In some States the insertion of fusible plugs at the highest fire line in boilers is compelled by law under a heavy penalty. Its design is to give the most emphatic warning of low water, and at the same time relieve the boiler of dangerous pressure.
Fig. 78.
Fig. 79.
Figs. [78] and [79] exhibit two of the forms most commonly used, and on the succeeding page, in [cut 80], is shown the device in operation where the water has sunk to a dangerously low level. In the illustration the device is shown in connection with a locomotive boiler, in the common tubular boiler the plug is usually inserted in the rear head of the boiler, so that in case of its operation it will not endanger the fireman.
These devices are designed to be screwed into the boiler shell at the safety line. The Figs. [78] & [79] exhibit their construction. The part to be screwed into the boiler is called the shell and is commonly made of brass; the internal part is plug and is made of a soft metal like banca tin or a compound consisting of lead, tin and bismuth. This composition melts easily at the proper point to allow escape, where the water has sunk to a dangerously low level.
There is considerable diversity in the make up of the material used for filling the plug, which must not have its melting point at anything less than the temperature of the steam lest it should “go off” at the wrong time.
